1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine, and more particularly to a spark plug adapted to jet plasma gases into a combustion chamber with the aid of self-induction electromagnetic force in the spark discharge to improve its ignitability.
2. Description of the Prior Art
In order to improve various performances of internal combustion engines, particularly gasoline engines, superior ignition performances must be ensured.
For example, with the extra lean mixture combustion system or exhaust gas recirculation system (EGR) which is considered to be effective to restrain the production of NOx to meet the strict exhaust gas regulation and limitation of poisonous material exhausts for automobile engines, how to reliably ignite the extra lean mixture or intake mixture including large amounts of exhaust gases is very important for ensuring required output performances.
For these purposes, various kinds of ignition systems have been proposed to improve the ignition performance. One of them is a spark plug utilizing electromagnetic force in spark discharge.
This spark plug comprises a main body 1 through which passes a center electrode 3 supported by an insulator 2 as shown in FIG. 1. The center electrode 3 has at its end a disc-like expansion 4 about which is formed a cylindrical earth electrode 6 with a discharging gap 5. There is provided a gas space 7 between the inside of the earth electrode 6 and the insulator 2.
With this spark plug 1, an electric current flows between the electrodes 6 and 4 by spark discharge to cause electromagnetic force which forces plasma gas (high temperature gas irons) produced in the gas space 7 in a spark discharging state into the combustion chamber to improve its ignitability or ignition performance.
Referring to FIG. 2, an electric current I flows from the earth electrode (positive electrode) 6 to the center electrode (negative electrode) 4 by the spark discharging to produce a magnetic field (whose magnetic flux density is Bo) in a clockwise direction about the current I flowing through the discharging gap 5. On the other hand, the current I flows through the center electrode 3 in its axial direction to cause a magnetic field (whose magnetic flux density is Bi) about the center electrode 3 in a clockwise direction.
The difference B(I) between the magnetic flux densities Bi and Bo of the inner and outer magnetic fields is indicated as B(I)=Bi-Bo&gt;0. The inner magnetic flux density Bi overcomes the outer flux density Bo, so that an electromagnetic force F is caused from the gap space 7 to the exterior of the plug 1 or the combustion chamber.
The electromagnetic force F is indicated by the following equation. ##EQU1## where r.sub.c is an outer radius of the center electrode 3 (expansion 4), r.sub.a is an inner radius of the earth electrode 6, J(I) is a current density and .mu. is a permeability.
This self-induction electromagnetic force F causes the high temperature plasma gas to rush into the center of the combustion chamber, thereby obtaining a very good ignition performance in comparison with the ignition in the proximity of a wall surface of a combustion chamber only by spark.
Moreover, it has been found that an electric potential in the order of several thousand volts is continuously supplied to such a spark plug at the same time of the spark discharge to perform the plasma ignition, thereby promoting the production and expansion of the plasma gas to obtain a better ignition performance.
In order to effectively produce the plasma gas with such a spark plug, however, a distance r.sub.a -r.sub.c for the discharging gap 5 must be large to a certain extent which would tend to produce electric waves of noises.
Namely, the larger the discharging gap 5, the higher a dielectric breakdown voltage is, and particularly with the plasma ignition, plasma energy with a great amount of electric current is emanated in sparking, so that there is a tendency for the ignition discharge to generate violent electric waves of noises. The wave noises generally disturb the broadcasting of radio and television and may give rise to serious troubles in electronic instruments loaded or equipped on a vehicle. How to restrain the wave noises is, therefore, an important problem in this field.
A spark plug widely used in automobile engines or the like has generally an air gap as a spark gap, so that a dielectric breakdown under a high compressive pressure is so high that an ignition device for generating high voltages as high as more than 10 KV is required with a tendency to emanate the electric waves of noises. To avoid this, high voltage resistance wires have been used for ignition cords. However, such wires unavoidably cause ignition energy losses to a certain extent.